The present invention relates to ellipsometry, wherein a beam with a known polarization state is focused on a sample and the change in polarization state after interaction with the sample is measured to determine characteristics of the sample. In particular, the present invention relates to systems and methodologies for mitigating errors induced in a focused beam spectroscopic ellipsometer employed in the evaluation of a samples such as semiconductor devices.
There is a need in the semiconductor as well as other industries to monitor compositional and geometrical features of a sample on a small scale. Various optical metrology tools have been developed to carry out this monitoring function including spectrophotometers and spectroscopic ellipsometers (SE).
Today, there is great interest in improving focused beam spectroscopic ellipsometers. In the semiconductor industry, measurements are typically made in the areas between devices on a wafer (streets). These streets are in effect wasted real estate on the wafer. As the capability improves to slice wafers, these streets are becoming smaller, necessitating even smaller focused beam spot sizes. In addition, there is current interest in measuring dimensional features such as critical dimensions in which a small focused spot is also desirable.
In a spectroscopic ellipsometer where the source beam is broadband, chromatic aberrations in the focusing optical elements limits the minimum achievable source spot size on the specimen. A larger spot size reduces the amount of light transported to the detector by the collection optics thus reducing throughput. Therefore, efforts are made to design systems with minimal chromatic aberration. If a refractive lens system is used to focus the beam, care must be taken to select the correct materials, curvatures and lens spacing to reduce chromatic aberrations. Another approach to reducing chromatic aberrations is to use curved mirrors to focus the beam.
In most commercial systems, the probe beam in an SE is polarized prior to being focused by either the refractive or reflective elements. However, if the source beam is polarized prior to being focused, polarization errors can be introduced. For example, the reflection from a focusing mirror can alter the polarization state of the source beam. When focusing a source beam with a lens or lens assembly, the birefringence in the lens materials tends to destabilize the polarization state. For example, a temperature change can adversely affect the polarization state of a polarized source beam before the beam is passed to the specimen. These effects introduce complications and sources of error into the analysis of the SE data.
This problem can be avoided by polarizing the source beam after it has been focused. Prior art attempts to achieve this result have included the use of a prism polarizer or a dichroic sheet polarizer. Unfortunately, dichroic sheet polarizers operate in only a limited spectral range. This is because dichroic sheet polarizers are polymer based. The polymers absorb ultraviolet, mid- and far- infrared radiation, which limits their use in the visible and near-infrared wavelengths. Prism polarizers are also problematic because they induce spherical aberration and chromatic aberration in a focused beam. This is not particular to prism polarizers, as aberrations may occur in any slab of material. Generally, as the thickness of the slab increases, the amount or severity of aberrations increases as well. However, because prism polarizers are relatively thick, the degree of aberrations formed is particularly more severe.
Therefore, it is desirable to accurately characterize a specimen without incurring such complications and sources of error by suitably polarizing the beam after focusing.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The present invention provides a novel optical measurement system and a methodology for mitigating and/or eliminating errors introduced into SE data that can be caused by a change in the polarization state of a polarized source beam focused by a mirror or lens. More specifically, the present invention involves polarizing the source beam after it has been focused in order to prevent or mitigate undesirable modifications to the polarization state of the polarized source beam. In addition, this specific approach reduces aberrations induced by polarizing a source beam after it has been focused. As a result, more light reaches the detector, thereby increasing the system throughput.
These results are accomplished in part by using a novel polarization system that is located between a focusing optical element and the specimen such that a source beam of light is polarized only after being focused. In particular, the polarization system may include at least one of a diffractive optical element and a wire grid polarizer in order to effectively polarize the source beam before being passed to the specimen. By way of example, the diffractive optical element may be used alone and/or added to one or more surfaces of a prism polarizer in order to compensate for aberrations introduced into the focused source beam by the prism polarizer. In addition, the polarization system may include a plurality of diffractive optical elements and/or wire grid polarizers such that the plurality of elements and/or grid polarizers is arranged in a sequence.
One aspect of the present invention relates to an optical measurement system that contains an SE measurement device. The system includes a light source, a polarization system, and a light detection system. The light source directs a source beam of light to a focusing optical element. Examples of the focusing optical elements include a focusing mirror and a focusing lens (e.g., lens assembly). The source beam is then polarized before reaching the specimen. Polarization of the source beam is performed by a polarization system, which is located between the focusing optical element and the specimen. After the source beam contacts the specimen and reflects therefrom, it is transmitted to the light detection system. The detection system determines the changes in polarization state of the source beam resulting from the interaction with the specimen.
Another aspect of the present invention relates to a polarization system employed within a SE. The polarization system contains one or more diffractive optical elements that are designed such that one polarization state has maximum diffraction efficiency in one order while the orthogonal polarization state has maximum efficiency in a different order. Therefore, the two orthogonal polarization states are separated into two beams that are focused to different locations on the specimen. Such an arrangement permits the use of spatial filtering (e.g., at the entrance aperture of the spectrometer) in order to transmit only the desired polarization state from the specimen to the detector. The polarization efficiency can be further increased by generating a phase grating on a birefringent substrate. Alternatively, or in addition, two such diffractive optical elements can be assembled to form a micro-array of Rochon prisms.
According to yet another aspect of the present invention, the polarization system may comprise a prism polarizer coupled to the diffractive optical element such that one or more diffractive elements may be added to one or more surfaces of a prism polarizer. The diffractive optical element configured in this manner compensates for aberrations induced by the prism polarizer.
More information on diffractive optical elements can be found in: xe2x80x9cAchromatic birefringent grating polarizer,xe2x80x9d S. Q. Liu, Y. S. Chen, R. X. Wang, Journal of modern optics. FEB 01, 1998 v 45 n 2335; xe2x80x9cPolarization device employing the combination effect of double refraction and diffraction,xe2x80x9d Shangqing Liu, Chengxiang Li and Yangsong Chen, Appl. Phys. Lett. 67 (1995) 1972; and xe2x80x9cMultilevel binary phase grating polarization device with a birefringent substrate,xe2x80x9d Shangqing Liu and Yansong Chen, Opt. Lett. 20 (1995)1832.
Yet another aspect of the present invention relates to a polarization system used in a SE measurement device wherein the polarization system contains a wire grid polarizer. The wire grid polarizer can act as a broadband dichroic sheet polarizer in the focused beam of the SE. Employing a wire grid polarizer allows one linear polarization state to be transmitted while the orthogonal polarization state can be either reflected or absorbed. This is accomplished in part by having a grid spacing smaller than the wavelength of the incident light (source beam). In addition, the effective medium combination of wire and adjacent areas may be linearly dichroic. Therefore, only a single linear polarization state is transmitted to the specimen.
Furthermore, the material of the wire grid substrate has a high transmittance in the spectral range that is to be detected. For example, suitable wire grid substrate materials have minimal imaginary parts of a dielectric response over the widest spectral ranges possible. In particular, for vacuum ultraviolet to infrared operation, suitable materials include calcium fluoride (CaF2), magnesium fluoride (MgF), aluminum oxide (Al2O3), lithium fluoride (LiF) and barium fluoride (BaF2). Suitable wire grid materials include, silver (Ag), gold (Au), aluminum (Al) and the like and other such materials that have maximal conductivity over the widest spectral ranges possible.
Still yet another aspect of the present invention relates to a polarization system employed within a measurement device such as a SE, wherein the polarization system includes a combination of a diffractive optical element and a wire grid polarizer.
Still yet another aspect of the present invention relates to a method for mitigating aberrations and distortions with respect to a focused source beam of an SE. The method involves directing a source beam of broadband light to a focusing optical element to focus the source beam. The focused source beam can be polarized by a polarization system. Subsequently, the polarized source beam is transmitted to a specimen. The source beam may include a plurality of wavelengths such as deep ultraviolet, infrared, and visible light. The focusing element may be a focusing mirror, lens, and/or lens assembly, depending on the desired application.
Moreover, mitigating the occurrence of aberrations introduced into the focused source beam facilitates obtaining a minimal source spot size on a specimen. As a result, one obtains simultaneously a single, well-defined polarization state of the source beam and a small source beam spot at the specimen, thereby facilitating evaluation and characterization of film properties of a semiconductor device.